I used a (Sena) case for my 3G, which I liked, except that I discovered that after a year or so, the back of my phone was scuffed up a lot more than my co-workers -- who never used any kind of case at all. The case was a magnetic flip-cover one, and also left odd marks on the font of the phone near where the magnets were.
So while it's silly of me to judge all cases by the failing of one, I'm going to avoid a case for the iPhone 4 for now. I might pick up one of the bumpers if I can see them in person (but I'm not going anywhere near the Apple store on launch day!). But part of the phone's appeal is its sleek look, for me, so hiding behind a case has never been something I was thrilled about.
Hopefully I won't drop it on day one. 😱